TNF-alpha regulates early differentiation of C2C12 myoblasts in an autocrine fashion

Tumor Necrosis Factor-Alpha Modulates Expression of Genes Involved in Cytokines and Chemokine Pathways in Proliferative Myoblast Cells

Skeletal muscle regeneration after injury is a complex process involving inflammatory signaling and myoblast activation. Pro-inflammatory cytokines like tumor necrosis factor-alpha (TNF-α) are key mediators, but their effects on gene expression in proliferating myoblasts are unclear. We performed the RNA sequencing of TNF-α treated C2C12 myoblasts to elucidate the signaling pathways and gene networks regulated by TNF-α during myoblast proliferation. The TNF-α (10 ng/mL) treatment of C2C12 cells led to 958 differentially expressed genes compared to the controls. Pathway analysis revealed significant regulation of TNF-α signaling, along with the chemokine and IL-17 pathways. Key upregulated genes included cytokines (e.g., IL-6), chemokines (e.g., CCL7), and matrix metalloproteinases (MMPs). TNF-α increased myogenic factor 5 (Myf5) but decreased MyoD protein levels and stimulated the release of MMP-9, MMP-10, and MMP-13. TNF-α also upregulates versican and myostatin mRNA. Overall, our study demonstrates the TNF-α modulation of distinct gene expression patterns and signaling pathways that likely contribute to enhanced myoblast proliferation while suppressing premature differentiation after muscle injury. Elucidating the mechanisms involved in skeletal muscle regeneration can aid in the development of regeneration-enhancing therapeutics.

Skeletal Muscle Injury in Chronic Kidney Disease-From Histologic Changes to Molecular Mechanisms and to Novel Therapies

Chronic kidney disease (CKD) is associated with significant reductions in lean body mass and in the mass of various tissues, including skeletal muscle, which causes fatigue and contributes to high mortality rates. In CKD, the cellular protein turnover is imbalanced, with protein degradation outweighing protein synthesis, leading to a loss of protein and cell mass, which impairs tissue function. As CKD itself, skeletal muscle wasting, or sarcopenia, can have various origins and causes, and both CKD and sarcopenia share common risk factors, such as diabetes, obesity, and age. While these pathologies together with reduced physical performance and malnutrition contribute to muscle loss, they cannot explain all features of CKD-associated sarcopenia. Metabolic acidosis, systemic inflammation, insulin resistance and the accumulation of uremic toxins have been identified as additional factors that occur in CKD and that can contribute to sarcopenia. Here, we discuss the elevation of systemic phosphate levels, also called hyperphosphatemia, and the imbalance in the endocrine regulators of phosphate metabolism as another CKD-associated pathology that can directly and indirectly harm skeletal muscle tissue. To identify causes, affected cell types, and the mechanisms of sarcopenia and thereby novel targets for therapeutic interventions, it is important to first characterize the precise pathologic changes on molecular, cellular, and histologic levels, and to do so in CKD patients as well as in animal models of CKD, which we describe here in detail. We also discuss the currently known pathomechanisms and therapeutic approaches of CKD-associated sarcopenia, as well as the effects of hyperphosphatemia and the novel drug targets it could provide to protect skeletal muscle in CKD.

The early inhibition of the COX-2 pathway in viperid phospholipase A2-induced skeletal muscle myotoxicity accelerates the tissue regeneration

The administration of non-steroidal anti-inflammatory drugs in the treatment of injury and muscle regeneration is still contradictory in effectiveness, especially regarding the timing of their administration. This can interfere with the production of prostaglandins originating from inflammatory isoform cyclooxygenase-2 (COX-2), which is essential to modulate tissue regeneration. The phospholipases A₂ (PLA₂) from viperid venoms cause myotoxicity, therefore constituting a tool for the study of supportive therapies to improve skeletal muscle regeneration. This study investigated the effect of early administration of lumiracoxib (selective inhibitor of COX-2) on the degeneration and regeneration stages of skeletal muscle after injury induced by a myotoxic PLA₂. After 30 min and 48 h of intramuscular injection of PLA₂, mice received lumiracoxib orally and histological, functional, and transcriptional parameters of muscle were evaluated from 6 h to 21 days. Inhibition of COX-2 in the early periods of PLA₂-induced muscle degeneration reduced leukocyte influx, edema, and tissue damage. After the second administration of lumiracoxib, in regenerative stage, muscle showed increase in number of basophilic fibers, reduction in fibrosis content and advanced recovery of functionality characterized by the presence of fast type II fibers. The expression of Pax7 and myogenin were increased, indicating a great capacity for storing satellite cells and advanced mature state of tissue. Our data reveals a distinct role of COX-2-derived products during muscle degeneration and regeneration, in which early administration of lumiracoxib was a therapeutic strategy to modulate the effects of prostaglandins, providing a breakthrough in muscle tissue regeneration induced by a myotoxic PLA_2.

Myogenic commitment of human stem cells by myoblasts Co-culture: a static vs. a dynamic approach

An in-vitro model of human bone marrow mesenchymal stem cells (hBM-MSCs) myogenic commitment by synergic effect of a differentiation media coupled with human primary skeletal myoblasts (hSkMs) co-culture was developed adopting both conventional static co-seeding and perfused culture systems. Static co-seeding provided a notable outcome in terms of gene expression with a significant increase of Desmin (141-fold) and Myosin heavy chain II (MYH2, 32-fold) at day 21, clearly detected also by semi-quantitative immunofluorescence. Under perfusion conditions, myogenic induction ability of hSkMs on hBM-MSCs was exerted by paracrine effect with an excellent gene overexpression and immunofluorescence detection of MYH2 protein; furthermore, due to the dynamic cell culture in separate wells, western blot data were acquired confirming a successful cell commitment at day 14. A significant increase of anti-inflammatory cytokine gene expression, including IL-10 and IL-4 (15-fold and 11-fold, respectively) at day 14, with respect to the pro-inflammatory cytokines IL-12A (7-fold at day 21) and IL-1β (1.4-fold at day 7) was also detected during dynamic culture, confirming the immunomodulatory activity of hBM-MSCs along with commitment events. The present study opens interesting perspectives on the use of dynamic culture based on perfusion as a versatile tool to study myogenic events and paracrine cross-talk compared to the simple co-seeding static culture.

Muscle cell-derived cytokines in skeletal muscle regeneration

Regeneration of the mammalian adult skeletal muscle is a well-orchestrated process regulated by multiple proteins and signalling pathways. Cytokines constitute a major class of regulators of skeletal myogenesis. It is well established that infiltrating immune cells at the site of muscle injury secrete cytokines, which play critical roles in the myofibre repair and regeneration process. In the past 10-15 years, skeletal muscle itself has emerged as a prolific producer of cytokines. Much attention in the field has been focused on the endocrine effects of muscle-secreted cytokines (myokines) on metabolic regulation. However, ample evidence suggests that muscle-derived cytokines also regulate myogenic differentiation and muscle regeneration in an autocrine manner. In this review, we survey cytokines that meet two criteria: (a) evidence of expression by muscle cells; (b) evidence demonstrating a myogenic function. Dozens of cytokines representing several major classes make up this group, and together they regulate all steps of the myogenic process. How such a large array of cytokines coordinate their signalling to form a regulatory network is a fascinating, pressing question. Functional studies that can distinguish the source of the cytokines in vivo are also much needed in order to facilitate exploration of their full therapeutic potential.

Paraspinal muscle imaging measurements for common spinal disorders: review and consensus-based recommendations from the ISSLS degenerative spinal phenotypes group

PURPOSE

Paraspinal muscle imaging is of growing interest related to improved phenotyping, prognosis, and treatment of common spinal disorders. We reviewed issues related to paraspinal muscle imaging measurement that contribute to inconsistent findings between studies and impede understanding.

METHODS

Three key contributors to inconsistencies among studies of paraspinal muscle imaging measurements were reviewed: failure to consider possible mechanisms underlying changes in paraspinal muscles, lack of control of confounding factors, and variations in spinal muscle imaging modalities and measurement protocols. Recommendations are provided to address these issues to improve the quality and coherence of future research.

RESULTS

Possible pathophysiological responses of paraspinal muscle to various common spinal disorders in acute or chronic phases are often overlooked, yet have important implications for the timing, distribution, and nature of changes in paraspinal muscle. These considerations, as well as adjustment for possible confounding factors, such as sex, age, and physical activity must be considered when planning and interpreting paraspinal muscle measurements in studies of spinal conditions. Adoption of standardised imaging measurement protocols for paraspinal muscle morphology and composition, considering the strengths and limitations of various imaging modalities, is critically important to interpretation and synthesis of research.

CONCLUSION

Study designs that consider physiological and pathophysiological responses of muscle, adjust for possible confounding factors, and use common, standardised measures are needed to advance knowledge of the determinants of variations or changes in paraspinal muscle and their influence on spinal health.

Tumor Necrosis Factor Alpha Signaling and Organogenesis

Tumor necrosis factor alpha (TNF-α) plays important roles in processes such as immunomodulation, fever, inflammatory response, inhibition of tumor formation, and inhibition of viral replication. TNF-α and its receptors are ubiquitously expressed in developing organs and they regulate the survival, proliferation, and apoptosis of embryonic stem cells (ESCs) and progenitor cells. TNF-α is an important inflammatory factor that also regulates the inflammatory response during organogenesis, and its cytotoxic effects can interfere with normal developmental processes, even leading to the onset of diseases. This review summarizes the various roles of TNF-α in organogenesis in terms of its secreting pattern, concentration-dependent activities, and interactions with other signaling pathways. We also explored new potential functions of TNF-α.

MiRNAs and Muscle Regeneration: Therapeutic Targets in Duchenne Muscular Dystrophy

microRNAs (miRNAs) are small non-coding RNAs required for the post-transcriptional control of gene expression. MicroRNAs play a critical role in modulating muscle regeneration and stem cell behavior. Muscle regeneration is affected in muscular dystrophies, and a critical point for the development of effective strategies for treating muscle disorders is optimizing approaches to target muscle stem cells in order to increase the ability to regenerate lost tissue. Within this framework, miRNAs are emerging as implicated in muscle stem cell response in neuromuscular disorders and new methodologies to regulate the expression of key microRNAs are coming up. In this review, we summarize recent advances highlighting the potential of miRNAs to be used in conjunction with gene replacement therapies, in order to improve muscle regeneration in the context of Duchenne Muscular Dystrophy (DMD).

Role of Sphingosine 1-Phosphate Signalling Axis in Muscle Atrophy Induced by TNFα in C2C12 Myotubes

Skeletal muscle atrophy is characterized by a decrease in muscle mass causing reduced agility, increased fatigability and higher risk of bone fractures. Inflammatory cytokines, such as tumor necrosis factor-alpha (TNFα), are strong inducers of skeletal muscle atrophy. The bioactive sphingolipid sphingosine 1-phoshate (S1P) plays an important role in skeletal muscle biology. S1P, generated by the phosphorylation of sphingosine catalyzed by sphingosine kinase (SK1/2), exerts most of its actions through its specific receptors, S1P_1-5. Here, we provide experimental evidence that TNFα induces atrophy and autophagy in skeletal muscle C2C12 myotubes, modulating the expression of specific markers and both active and passive membrane electrophysiological properties. NMR-metabolomics provided a clear picture of the deep remodelling of skeletal muscle fibre metabolism induced by TNFα challenge. The cytokine is responsible for the modulation of S1P signalling axis, upregulating mRNA levels of S1P₂ and S1P₃ and downregulating those of SK2. TNFα increases the phosphorylated form of SK1, readout of its activation. Interestingly, pharmacological inhibition of SK1 and specific antagonism of S1P₃ prevented the increase in autophagy markers and the changes in the electrophysiological properties of C2C12 myotubes without affecting metabolic remodelling induced by the cytokine, highlighting the involvement of S1P signalling axis on TNFα-induced atrophy in skeletal muscle.

IL-1β and TNF-α Modulation of Proliferated and Committed Myoblasts: IL-6 and COX-2-Derived Prostaglandins as Key Actors in the Mechanisms Involved

In this study, we investigated the effects and mechanisms of the pro-inflammatory cytokines IL-1β and TNF-α on the proliferation and commitment phases of myoblast differentiation. C2C12 mouse myoblast cells were cultured to reach a proliferated or committed status and were incubated with these cytokines for the evaluation of cell proliferation, cyclooxygenase 2 (COX-2) expression, release of prostaglandins (PGs) and myokines, and activation of myogenic regulatory factors (MRFs). We found that inhibition of the IL-6 receptor reduced IL-1β- and TNF-α-induced cell proliferation, and that the IL-1β effect also involved COX-2-derived PGs. Both cytokines modulated the release of the myokines myostatin, irisin, osteonectin, and IL-15. TNF-α and IL-6 reduced the activity of Pax7 in proliferated cells and reduced MyoD and myogenin activity at both proliferative and commitment stages. Otherwise, IL-1β increased myogenin activity only in committed cells. Our data reveal a key role of IL-6 and COX-2-derived PGs in IL-1β and TNF-α-induced myoblast proliferation and support the link between TNF-α and IL-6 and the activation of MRFs. We concluded that IL-1β and TNF-α induce similar effects at the initial stages of muscle regeneration but found critical differences between their effects with the progression of the process, bringing new insights into inflammatory signalling in skeletal muscle regeneration.

RG, Adler KA, Cook G, Molina PE, Simon L. Chronic Alcohol Dysregulates Skeletal Muscle Myogenic Gene Expression after Hind Limb Immobilization in Female Rats

Alcohol use and aging are risk factors for falls requiring immobilization and leading to skeletal muscle atrophy. Skeletal muscle regeneration is integral to post-immobilization recovery. This study aimed to elucidate the effects of alcohol and ovarian hormone loss on the expression of genes implicated in muscle regeneration. Three-month-old female rats received an ovariectomy or a sham surgery, consumed an alcohol-containing or control diet for 10 weeks, were subjected to unilateral hind limb immobilization for seven days, and finally were allowed a three (3d)- or 14 (14d)-day recovery. Immobilization decreased the quadriceps weight at 3d and 14d, and alcohol decreased the quadriceps weight at 14d in the nonimmobilized hind limb (NI). At 3d, alcohol decreased gene expression of myoblast determination protein (MyoD) in the immobilized hind limb (IMM) and myocyte enhancer factor (Mef)2C and tumor necrosis factor (TNF)α in NI, and ovariectomy increased MyoD and decreased TNFα expression in NI. At 14d, alcohol increased the gene expression of Mef2C, MyoD, TNFα, and transforming growth factor (TFG)β in IMM and decreased monocyte chemoattractant protein (MCP)1 expression in NI; ovariectomy increased TNFα expression in NI, and alcohol and ovariectomy together increased Mef2C expression in NI. Despite increased TGFβ expression, there was no concomitant alcohol-mediated increase in collagen in IMM at 14d. Overall, these data indicate that alcohol dysregulated the post-immobilization alteration in the expression of genes implicated in regeneration. Whether alcohol-mediated molecular changes correspond with post-immobilization functional alterations remains to be determined.

TAK-242, a specific inhibitor of Toll-like receptor 4 signalling, prevents endotoxemia-induced skeletal muscle wasting in mice

Circulating lipopolysaccharide (LPS) concentrations are often elevated in patients with sepsis or various endogenous diseases related to bacterial translocation from the gut. Systemic inflammatory responses induced by endotoxemia induce severe involuntary loss of skeletal muscle, termed muscle wasting, which adversely affects the survival and functional outcomes of these patients. Currently, no drugs are available for the treatment of endotoxemia-induced skeletal muscle wasting. Here, we tested the effects of TAK-242, a Toll-like receptor 4 (TLR4)-specific signalling inhibitor, on myotube atrophy in vitro and muscle wasting in vivo induced by endotoxin. LPS treatment of murine C2C12 myotubes induced an inflammatory response (increased nuclear factor-κB activity and interleukin-6 and tumour necrosis factor-α expression) and activated the ubiquitin-proteasome and autophagy proteolytic pathways (increased atrogin-1/MAFbx, MuRF1, and LC-II expression), resulting in myotube atrophy. In mice, LPS injection increased the same inflammatory and proteolytic pathways in skeletal muscle and induced atrophy, resulting in reduced grip strength. Notably, pretreatment of cells or mice with TAK-242 reduced or reversed all the detrimental effects of LPS in vitro and in vivo. Collectively, our results indicate that pharmacological inhibition of TLR4 signalling may be a novel therapeutic intervention for endotoxemia-induced muscle wasting.

Eccentric exercise results in a prolonged increase in interleukin-6 and tumor necrosis factor-α levels in rat skeletal muscle

Interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) are well-known cytokines with pro-inflammatory capabilities, and have been shown to be involved in adaptation to exercise as multifaceted myokines. However, the precise role of IL-6 and TNF-α during exercise-induced skeletal muscle injury and subsequent repair processes is not fully understood. In this study, IL-6 and TNF-α were examined in soleus muscles at the gene and protein levels using in situ hybridization and immunohistochemical staining, respectively, and serum levels of IL-6 and TNF-α were determined before and after a 90-min downhill running session in rats. There were no changes in serum levels of IL-6 and TNF-α after exercise, but IL-6 and TNF-α mRNA increased and maintained high expression in muscles for 1-2 weeks after exercise. IL-6 and TNF-a mRNAs were identified in both the cytoplasm and the nuclei of myocytes, as well as in invading inflammatory cells. IL-6 and TNF-α protein mainly distributed in cytoplasm unevenly and had a prolonged expression until 2 weeks after eccentric exercise. Our results demonstrate that there is increased IL-6 and TNF-α expression in skeletal muscle that is induced by eccentric exercise and that the high expression of IL-6 and TNF-α in the long-term phase after eccentric exercise may be more involved in the subsequent recovery of damaged muscle.

Changes in Structure and Function of the Back Muscles in Low Back Pain: Different Time Points, Observations, and Mechanisms

Spinal health depends on optimal back muscle performance, and this is determined by muscle structure and function. There has been substantial research evaluating the differences in structure and function of many back muscles, including the multifidus and erector spinae, but with considerable variation in results. Many studies have shown atrophy, fat infiltration, and connective tissue accumulation in back muscles, particularly deep fibers of the multifidus, but the results are not uniform. In terms of function, results are also somewhat inconsistent, often reporting lower multifidus activation and augmented recruitment of more superficial components of the multifidus and erector spinae, but, again, with variation between studies. A major recent observation has been the identification of time-dependent differences in features of back muscle adaptation, from acute to subacute/recurrent to chronic states of the condition. Further, these adaptations have been shown to be explained by different time-dependent mechanisms. This has substantial impact on the rationale for rehabilitation approaches. The aim of this commentary was to review and consolidate the breadth of research investigating adaptation in back muscle structure and function, to consider explanations for some of the variation between studies, and to propose how this model can be used to guide rehabilitation in a manner that is tailored to individual patients and to underlying mechanisms. J Orthop Sports Phys Ther 2019;49(6):464-476. doi:10.2519/jospt.2019.8827.

Bradykinin mediates myogenic differentiation in murine myoblasts through the involvement of SK1/Spns2/S1P2 axis

Skeletal muscle tissue retains a remarkable regenerative capacity due to the activation of resident stem cells that in pathological conditions or after tissue damage proliferate and commit themselves into myoblasts. These immature myogenic cells undergo differentiation to generate new myofibers or repair the injured ones, giving a strong contribution to muscle regeneration. Cytokines and growth factors, potently released after tissue injury by leukocytes and macrophages, are not only responsible of the induction of the initial inflammatory response, but can also affect skeletal muscle regeneration. Growth factors exploit sphingosine kinase (SK), the enzyme that catalyzes the production of sphingosine 1-phosphate (S1P), to exert their biological effects in skeletal muscle. In this paper we show for the first time that bradykinin (BK), the leading member of kinin/kallikrein system, is able to induce myogenic differentiation in C2C12 myoblasts. Moreover, evidence is provided that SK1, the specific S1P-transporter spinster homolog 2 (Spns2) and S1P₂ receptor are involved in the action exerted by BK, since pharmacological inhibition/antagonism or specific down-regulation significantly alter BK-induced myogenic differentiation. Moreover, the molecular mechanism initiated by BK involves a rapid translocation of SK1 to plasma membrane, analyzed by time-lapse immunofluorescence analysis. The present study highlights the role of SK1/Spns2/S1P receptor 2 signaling axis in BK-induced myogenic differentiation, thus confirming the crucial involvement of this pathway in skeletal muscle cell biology.

Lipopolysaccharide inhibits myogenic differentiation of C2C12 myoblasts through the Toll-like receptor 4-nuclear factor-κB signaling pathway and myoblast-derived tumor necrosis factor-α

Background

Circulating lipopolysaccharide (LPS) concentrations are often elevated in patients with sepsis or with various endogenous diseases that are associated with metabolic endotoxemia. Involuntary loss of skeletal muscle, termed muscle wasting, is commonly observed in these conditions, suggesting that circulating LPS might play an essential role in its development. Although impairment of muscle regeneration is an important determinant of skeletal muscle wasting, it is unclear whether LPS affects this process and, if so, by what mechanism. Here, we used the C2C12 myoblast cell line to investigate the effects of LPS on myogenesis.

Methods

C2C12 myoblasts were grown to 80% confluence and induced to differentiate in the absence or presence of LPS (0.1 or 1 μg/mL); TAK-242 (1 μM), a specific inhibitor of Toll-like receptor 4 (TLR4) signaling; and a tumor necrosis factor (TNF)-α neutralizing antibody (5 μg/mL). Expression of a skeletal muscle differentiation marker (myosin heavy chain II), two essential myogenic regulatory factors (myogenin and MyoD), and a muscle negative regulatory factor (myostatin) was analyzed by western blotting. Nuclear factor-κB (NF-κB) DNA-binding activity was measured using an enzyme-linked immunosorbent assay.

Results

LPS dose-dependently and significantly decreased the formation of multinucleated myotubes and the expression of myosin heavy chain II, myogenin, and MyoD, and increased NF-κB DNA-binding activity and myostatin expression. The inhibitory effect of LPS on myogenic differentiation was reversible, suggesting that it was not caused by nonspecific toxicity. Both TAK-242 and anti-TNF-α reduced the LPS-induced increase in NF-κB DNA-binding activity, downregulation of myogenic regulatory factors, and upregulation of myostatin, thereby partially rescuing the impairment of myogenesis.

Conclusions

Our data suggest that LPS inhibits myogenic differentiation via a TLR4–NF-κB-dependent pathway and an autocrine/paracrine TNF-α-induced pathway. These pathways may be involved in the development of muscle wasting caused by sepsis or metabolic endotoxemia.

Cell geometry dictates TNFα-induced genome response

Cells in physiology integrate local soluble and mechanical signals to regulate genomic programs. Whereas the individual roles of these signals are well studied, the cellular responses to the combined chemical and physical signals are less explored. Here, we investigated the cross-talk between cellular geometry and TNFα signaling. We stabilized NIH 3T3 fibroblasts into rectangular anisotropic or circular isotropic geometries and stimulated them with TNFα and analyzed nuclear translocation of transcription regulators -NFκB (p65) and MKL and downstream gene-expression patterns. We found that TNFα induces geometry-dependent actin depolymerization, which enhances IκB degradation, p65 nuclear translocation, nuclear exit of MKL, and sequestration of p65 at the RNA-polymerase-II foci. Further, global transcription profile of cells under matrix-TNFα interplay reveals a geometry-dependent gene-expression pattern. At a functional level, we find cell geometry affects TNFα-induced cell proliferation. Our results provide compelling evidence that fibroblasts, depending on their geometries, elicit distinct cellular responses for the same cytokine.

Genome Engineering of Stem Cells for Autonomously Regulated, Closed-Loop Delivery of Biologic Drugs

Chronic inflammatory diseases such as arthritis are characterized by dysregulated responses to pro-inflammatory cytokines such as interleukin-1 (IL-1) and tumor necrosis factor α (TNF-α). Pharmacologic anti-cytokine therapies are often effective at diminishing this inflammatory response but have significant side effects and are used at high, constant doses that do not reflect the dynamic nature of disease activity. Using the CRISPR/Cas9 genome-engineering system, we created stem cells that antagonize IL-1- or TNF-α-mediated inflammation in an autoregulated, feedback-controlled manner. Our results show that genome engineering can be used successfully to rewire endogenous cell circuits to allow for prescribed input/output relationships between inflammatory mediators and their antagonists, providing a foundation for cell-based drug delivery or cell-based vaccines via a rapidly responsive, autoregulated system. The customization of intrinsic cellular signaling pathways in stem cells, as demonstrated here, opens innovative possibilities for safer and more effective therapeutic approaches for a wide variety of diseases.

NF-κB signalling and cell fate decisions in response to a short pulse of tumour necrosis factor

In tissues and tumours, cell behaviours are regulated by multiple time-varying signals. While in the laboratory cells are often exposed to a stimulus for the duration of the experiment, in vivo exposures may be much shorter. In this study, we monitored NF-κB and caspase signalling in human cancer cells treated with a short pulse of Tumour Necrosis Factor (TNF). TNF is an inflammatory cytokine that can induce both the pro-survival NF-κB-driven gene transcription pathway and the pro-apoptotic caspase pathway. We find that a few seconds of exposure to TNF is sufficient to activate the NF-κB pathway in HeLa cells and induce apoptotic cell death in both HeLa and Kym-1 cells. Strikingly, a 1-min pulse of TNF can be more effective at killing than a 1-hour pulse, indicating that in addition to TNF concentration, duration of exposure also coordinates cell fate decisions.

Effects of Biowastes Released by Mechanically Damaged Muscle Cells on the Propagation of Deep Tissue Injury: A Multiphysics Study

Deep tissue injuries occur in muscle tissues around bony prominences under mechanical loading leading to severe pressure ulcers. Tissue compression can potentially compromise lymphatic transport and cause accumulation of metabolic biowastes, which may cause further cell damage under continuous mechanical loading. In this study, we hypothesized that biowastes released by mechanically damaged muscle cells could be toxic to the surrounding muscle cells and could compromise the capability of the surrounding muscle cells to withstand further mechanical loadings. In vitro, we applied prolonged low compressive stress (PLCS) and short-term high compressive stress to myoblasts to cause cell damage and collected the biowastes released by the damaged cells under the respective loading scenarios. In silico, we used COMSOL to simulate the compressive stress distribution and the diffusion of biowastes in a semi-3D buttock finite element model. In vitro results showed that biowastes collected from cells damaged under PLCS were more toxic and could compromise the capability of normal myoblasts to resist compressive damage. In silico results showed that higher biowastes diffusion coefficient, higher biowastes release rate, lower biowastes tolerance threshold and earlier timeline of releasing biowastes would cause faster propagation of tissue damage. This study highlighted the importance of biowastes in the development of deep tissue injury to clinical pressure ulcers under prolonged skeletal compression.

Ion Current-Based Proteomic Profiling for Understanding the Inhibitory Effect of Tumor Necrosis Factor Alpha on Myogenic Differentiation

Despite a demonstrated role for TNF-α in promoting muscle wasting and cachexia, the associated molecular mechanisms and signaling pathways of myoblast differentiation dysregulated by TNF-α remain poorly understood. This study presents well-controlled proteomic profiling as a means to investigate the mechanisms of TNF-α-regulated myogenic differentiation. Primary human muscle precursor cells (MPCs) cultured in growth medium (GM), differentiation medium (DM) to induce myogenic differentiation, and DM with 20 ng/mL of TNF-α (n = 5/group) were comparatively analyzed by an ion current-based quantitative platform consisting of reproducible sample preparation/on-pellet digestion, a long-column nano-LC separation, and ion current-based differential analysis. The inhibition of myogenic differentiation by TNF-α was confirmed by reduced formation of multinucleated myotubes and the recovered expression of altered myogenic proteins such as MYOD and myogenin during myogenic differentiation. Functional analysis and validation by immunoassay analysis suggested that the cooperation of NF-κB and STAT proteins is responsible for dysregulated differentiation in MPCs by TNF-α treatment. Increased MHC class I components such as HLA-A, HLA-B, HLA-C, and beta-2-microglobulin were also observed in cultures in DM treated with TNF-α. Interestingly, inhibition of the cholesterol biosynthesis pathway during myogenic differentiation induced by serum starvation was not recovered by TNF-α treatment, which combined with previous reports, implies that this process may be an early event of myogenesis. This finding could lay the foundation for the potential use of statins in modulating myogenesis through cholesterol, for example, in stem cell-based myocardial infarction treatment, where differentiation of myoblasts and stem cells into force-generating mature muscle cells is a key step to the therapeutic capacity. In conclusion, the landscapes of altered transcription regulators, metabolic processes, and signaling pathways in MPCs are revealed in the regulation of myogenic differentiation by TNF-α, which is valuable for myogenic cellular therapeutics.

Mesenchymal Stem Cell Treatment of Intervertebral Disc Lesion Prevents Fatty Infiltration and Fibrosis of the Multifidus Muscle, but not Cytokine and Muscle Fiber Changes

STUDY DESIGN

Longitudinal case-control animal model.

OBJECTIVE

To investigate effects of mesenchymal stem cell (MSC) treatment on multifidus muscle remodeling after intervertebral disc (IVD) lesion.

SUMMARY OF BACKGROUND DATA

Lesion and degeneration of IVDs cause structural remodeling of the multifidus muscle. Proinflammatory cytokines are thought to contribute. MSC treatment restores IVD health after lesion but its effects on surrounding tissues remains unknown. Using an animal model of IVD degeneration, we assessed the effects of MSC treatment of IVDs on the structural remodeling and cytokine expression within the multifidus muscle.

METHODS

An anterolateral lesion was performed on the L1-2, L3-4, and L5-6 IVDs in sheep. At either 4 (early treatment) or 12 (late treatment) weeks after IVD lesion, MSCs were injected into the lesioned IVD. Multifidus muscle was harvested from L2 (gene expression analysis) and L4 (histological analysis) at 3 or 6 months after IVD lesion and naïve controls for histological analysis of muscle, adipose, and connective tissue cross-sectional areas, and immunohistochemistry to study muscle fiber types. Real-time polymerase chain reactions quantified expression of tumor necrosis factor, interleukin-1β, and transforming growth factor-β1.

RESULTS

MSC treatment of IVD lesion prevented the increased adipose and connective tissue cross-sectional area expected after IVD lesion. MSC treatment did not prevent slow-to-fast muscle fiber type transformation. Gene expression of proinflammatory cytokines within the muscle was altered by the MSC treatment of IVD. Increased interleukin-1β expression was prevented in the early treatment group and tumor necrosis factor and transforming growth factor-β1 expression was upregulated at 6 months.

CONCLUSION

Results show that although MSC treatment prevents fatty infiltration and fibrosis of the multifidus muscle after IVD lesion, it cannot prevent a muscle inflammatory response and muscle fiber transformation. These findings highlight the potential role of MSC therapy after IVD injury, but reveals that other interventions may also be necessary to optimize recovery of muscle.

LEVEL OF EVIDENCE

4.

Co- and post-transcriptional regulation of Rbm5 and Rbm10 in mouse cells as evidenced by tissue-specific, developmental and disease-associated variation of splice variant and protein expression levels

BACKGROUND

Expression and function of the two RNA binding proteins and regulators of alternative splicing, RBM5 and RBM10, have largely been studied in human tissue and cell lines. The objective of the study described herein was to examine their expression in mouse tissue, in order to lay the framework for comprehensive functional studies using mouse models.

METHODS

All RNA variants of Rbm5 and Rbm10 were examined in a range of normal primary mouse tissues. RNA and protein were examined in differentiating C2C12 myoblasts and in denervated and dystonin-deficient mouse skeletal muscle.

RESULTS

All Rbm5 and Rbm10 variants examined were expressed in all mouse tissues and cell lines. In general, Rbm5 and Rbm10 RNA expression was higher in brain than in skin. RNA expression levels were more varied between cardiac and skeletal muscle, depending on the splice variant: for instance, Rbm10v1 RNA was higher in skeletal than cardiac muscle, whereas Rbm10v3 RNA was higher in cardiac than skeletal muscle. In mouse brain, cardiac and skeletal muscle, RNA encoding an approximately 17kDa potential paralogue of a small human RBM10 isoform was detected, and the protein observed in myoblasts and myotubes. Expression of Rbm5 and Rbm10 RNA remained constant during C2C12 myogenesis, but protein levels significantly decreased. In two muscle disease models, neither Rbm10 nor Rbm5 showed significant transcriptional changes, although significant specific alternative splicing changes of Rbm5 pre-mRNA were observed. Increased RBM10 protein levels were observed following denervation.

CONCLUSIONS

The varied co-transcriptional and post-transcriptional regulation aspects of Rbm5 and Rbm10 expression associated with mouse tissues, myogenesis and muscle disease states suggest that a mouse model would be an interesting and useful model in which to study comprehensive functional aspects of RBM5 and RBM10.

Aryl hydrocarbon receptor-dependent apoptotic cell death induced by the flavonoid chrysin in human colorectal cancer cells

The polyphenolic flavone chrysin has been evaluated as a natural chemopreventive agent due to its anti-cancer effects in a variety of cancer cell lines. However, the mechanism of the chemopreventive effect has been not well established, especially in human colorectal cancer cells. We evaluated the chemopreventive effect of chrysin in three different human colorectal cancer cell lines. We found that chrysin treatment consequently reduced cell viability via induction of apoptosis. We identified that the involvement of up-regulation of pro-apoptotic cytokines tumor necrosis factor (Tnf) α and β genes and consequent activation of the TNF-mediated transcriptional pathway in chrysin-induced apoptosis. Using our generated AHR siRNA expressing colorectal cancer cells, we demonstrated that the chrysin-induced up-regulation of Tnfα and β gene expression was dependent on the aryl hydrocarbon receptor (AHR), which is a ligand-receptor for chrysin. Subsequently, we found that the AHR siRNA expressing colorectal cancer cells were resistant to chrysin-induced apoptosis. Therefore, we concluded that AHR is required for the chrysin-induced apoptosis and the up-regulation of Tnfα and β gene expression in human colorectal cancer cells.

Phosphorylation of αB-crystallin: Role in stress, aging and patho-physiological conditions

BACKGROUND

αB-crystallin, once thought to be a lenticular protein, is ubiquitous and has critical roles in several cellular processes that are modulated by phosphorylation. Serine residues 19, 45 and 59 of αB-crystallin undergo phosphorylation. Phosphorylation of S45 is mediated by p44/42 MAP kinase, whereas S59 phosphorylation is mediated by MAPKAP kinase-2. Pathway involved in S19 phosphorylation is not known.

SCOPE OF REVIEW

The review highlights the role of phosphorylation in (i) oligomeric structure, stability and chaperone activity, (ii) cellular processes such as apoptosis, myogenic differentiation, cell cycle regulation and angiogenesis, and (iii) aging, stress, cardiomyopathy-causing αB-crystallin mutants, and in other diseases.

MAJOR CONCLUSIONS

Depending on the context and extent of phosphorylation, αB-crystallin seems to confer beneficial or deleterious effects. Phosphorylation alters structure, stability, size distribution and dynamics of the oligomeric assembly, thus modulating chaperone activity and various cellular processes. Phosphorylated αB-crystallin has a tendency to partition to the cytoskeleton and hence to the insoluble fraction. Low levels of phosphorylation appear to be protective, while hyperphosphorylation has negative implications. Mutations in αB-crystallin, such as R120G, Q151X and 464delCT, associated with inherited myofibrillar myopathy lead to hyperphosphorylation and intracellular inclusions. An ongoing study in our laboratory with phosphorylation-mimicking mutants indicates that phosphorylation of R120GαB-crystallin increases its propensity to aggregate.

GENERAL SIGNIFICANCE

Phosphorylation of αB-crystallin has dual role that manifests either beneficial or deleterious consequences depending on the extent of phosphorylation and interaction with cytoskeleton. Considering that disease-causing mutants of αB-crystallin are hyperphosphorylated, moderation of phosphorylation may be a useful strategy in disease management. This article is part of a Special Issue entitled Crystallin Biochemistry in Health and Disease.

Mouse Cd59b but not Cd59a is upregulated to protect cells from complement attack in response to inflammatory stimulation

Universally expressed CD59 is the sole membrane complement regulatory protein that protects host cells from complement damage by restricting membrane attack complex assembly. The human gene encodes a single CD59, whereas the mouse gene encodes a duplicated CD59, comprising mCd59a and mCd59b, with distinct tissue distribution. Recently, we revealed that Sp1 regulates constitutive CD59 transcription and that canonical nuclear factor kappa light chain enhancer of activated B cells (NF-κB) and cyclic AMP-responsive element-binding protein (CREB) regulate inducible CD59 transcription. However, the mechanisms that underlie mCd59 regulation remain unclear. Here we demonstrate that Sp1 controls broadly distributed mCd59a expression, whereas serum response factor (SRF) and canonical NF-κB regulate selectively expressed mCd59b. Tumor necrosis factor-α in vitro and lipopolysaccharide in vivo remarkably enhance the expression of mCd59b but not mCd59a by activating SRF and NF-κB, thus protecting cells from complement attack. In addition, cAMP analog treatment also dramatically increases mCd59b but not mCd59a expression in a manner independent of CREB, SRF and NF-κB. Therefore, mCd59b but not mCd59a may be the responder to external inflammatory stimuli and may have an important role in complement-mediated mouse models of disease.

Multifidus Muscle Changes After Back Injury Are Characterized by Structural Remodeling of Muscle, Adipose and Connective Tissue, but Not Muscle Atrophy: Molecular and Morphological Evidence

STUDY DESIGN

Longitudinal case-controlled animal study.

OBJECTIVE

To investigate putative cellular mechanisms to explain structural changes in muscle and adipose and connective tissues of the back muscles after intervertebral disc (IVD) injury.

SUMMARY OF BACKGROUND DATA

Structural back muscle changes are ubiquitous with back pain/injury and considered relevant for outcome, but their exact nature, time course, and cellular mechanisms remain elusive. We used an animal model that produces phenotypic back muscle changes after IVD injury to study these issues at the cellular/molecular level.

METHODS

Multifidus muscle was harvested from both sides of the spine at L1-L2 and L3-L4 IVDs in 27 castrated male sheep at 3 (n = 10) or 6 (n = 17) months after a surgical anterolateral IVD injury at both levels. Ten control sheep underwent no surgery (3 mo, n = 4; 6 mo, n = 6). Tissue was harvested at L4 for histological analysis of cross-sectional area of muscle and adipose and connective tissue (whole muscle), plus immunohistochemistry to identify proportion and cross-sectional area of individual muscle fiber types in the deepest fascicle. Quantitative polymerase chain reaction measured gene expression of typical cytokines/signaling molecules at L2.

RESULTS

Contrary to predictions, there was no multifidus muscle atrophy (whole muscle or individual fiber). There was increased adipose and connective tissue (fibrotic proliferation) cross-sectional area and slow-to-fast muscle fiber transition at 6 but not 3 months. Within the multifidus muscle, increases in the expression of several cytokines (tumor necrosis factor α and interleukin-1β) and molecules that signal trophic/atrophic processes for the 3 tissue types (e.g., growth factor pathway [IGF-1, PI3k, Akt1, mTOR], potent tissue modifiers [calcineurin, PCG-1α, and myostatin]) were present.

CONCLUSION

This study provides cellular evidence that refutes the presence of multifidus muscle atrophy accompanying IVD degeneration at this intermediate time point. Instead, adipose/connective tissue increased in parallel with the expression of the genes that provide putative mechanisms for multifidus structural remodeling. This provides novel targets for pharmacological and physical interventions.

LEVEL OF EVIDENCE

N/A.

Small heat shock proteins: Role in cellular functions and pathology

Small heat shock proteins (sHsps) are conserved across species and are important in stress tolerance. Many sHsps exhibit chaperone-like activity in preventing aggregation of target proteins, keeping them in a folding-competent state and refolding them by themselves or in concert with other ATP-dependent chaperones. Mutations in human sHsps result in myopathies, neuropathies and cataract. Their expression is modulated in diseases such as Alzheimer's, Parkinson's and cancer. Their ability to bind Cu2+, and suppress generation of reactive oxygen species (ROS) may have implications in Cu2+-homeostasis and neurodegenerative diseases. Circulating αB-crystallin and Hsp27 in the plasma may exhibit immunomodulatory and anti-inflammatory functions. αB-crystallin and Hsp20 exhitbit anti-platelet aggregation: these beneficial effects indicate their use as potential therapeutic agents. sHsps have roles in differentiation, proteasomal degradation, autophagy and development. sHsps exhibit a robust anti-apoptotic property, involving several stages of mitochondrial-mediated, extrinsic apoptotic as well as pro-survival pathways. Dynamic N- and C-termini and oligomeric assemblies of αB-crystallin and Hsp27 are important factors for their functions. We propose a "dynamic partitioning hypothesis" for the promiscuous interactions and pleotropic functions exhibited by sHsps. Stress tolerance and anti-apoptotic properties of sHsps have both beneficial and deleterious consequences in human health and diseases. Conditional and targeted modulation of their expression and/or activity could be used as strategies in treating several human disorders. The review attempts to provide a critical overview of sHsps and their divergent roles in cellular processes particularly in the context of human health and disease.

Genetic deletion of TNFR2 augments inflammatory response and blunts satellite-cell-mediated recovery response in a hind limb ischemia model

We have previously shown that TNF-tumor necrosis factor receptor-2/p75 (TNFR2/p75) signaling plays a critical role in ischemia-induced neovascularization in skeletal muscle and heart tissues. To determine the role of TNF-TNFR2/p75 signaling in ischemia-induced inflammation and muscle regeneration, we subjected wild-type (WT) and TNFR2/p75 knockout (p75KO) mice to hind limb ischemia (HLI) surgery. Ischemia induced significant and long-lasting inflammation associated with considerable decrease in satellite-cell activation in p75KO muscle tissue up to 10 d after HLI surgery. To determine the possible additive negative roles of tissue aging and the absence of TNFR2/p75, either in the tissue or in the bone marrow (BM), we generated 2 chimeric BM transplantation (BMT) models where both young green fluorescent protein (GFP)-positive p75KO and WT BM-derived cells were transplanted into adult p75KO mice. HLI surgery was performed 1 mo after BMT, after confirming complete engraftment of the recipient BM with GFP donor cells. In adult p75KO with the WT-BMT, proliferative (Ki67(+)) cells were detected only by d 28 and were exclusively GFP(+), suggesting significantly delayed contribution of young WT-BM cell to adult p75KO ischemic tissue recovery. No GFP(+) young p75KO BM cells survived in adult p75KO tissue, signifying the additive negative roles of tissue aging combined with decreased/absent TNFR2/p75 signaling in postischemic recovery.

Can proinflammatory cytokine gene expression explain multifidus muscle fiber changes after an intervertebral disc lesion?

STUDY DESIGN

Longitudinal case-controlled animal study.

OBJECTIVE

To investigate the effect of an intervertebral disc (IVD) lesion on the proportion of slow, fast, and intermediate muscle fiber types in the multifidus muscle in sheep, and whether muscle fiber changes were paralleled by local gene expression of the proinflammatory cytokines tumor necrosis factor α (TNF-α) and interleukin 1-β.

SUMMARY OF BACKGROUND DATA

Structure and behavior of the multifidus muscle change in acute and chronic back pain, but the mechanisms are surprisingly poorly understood and the link between structure and behavior is tenuous. Although changes in muscle fiber types have the potential to unify the observations, the effect of injury on muscle fiber distribution has not been adequately tested, and understanding of possible mechanisms is limited.

METHODS

The L1-L2, L3-L4, and L5-L6 IVDs of 11 castrated male sheep received anterolateral lesions. Six control sheep underwent no surgical procedures. Multifidus muscle tissue was harvested at L4 for muscle fiber analysis using immunohistochemistry and L2 for cytokine analysis with polymerase chain reaction for local gene expression of TNF-α and interleukin-1β.

RESULTS

The proportion of slow muscle fibers in multifidus was significantly less in the lesioned animals both ipsilateral and contralateral to the IVD lesion. The greatest reduction in slow fibers was in the deep medial muscle region. A greater prevalence of intermediate fibers on the uninjured side implies a delayed fiber-type transformation on that side. TNF-α gene expression in multifidus was greater on both sides in the lesion animals than in the muscle of control animals. Interleukin-1β was increased only on the injured side.

CONCLUSION

These data provide evidence of muscle fiber changes after induction of an IVD lesion and a parallel increase in TNF-α expression. Proinflammatory cytokine changes provide a novel mechanism to explain behavioral and structural changes in multifidus.

LEVEL OF EVIDENCE

N/A.

Regulation of myogenic activation of p38 MAPK by TACE-mediated TNFα release

The activation of p38 MAPK in myogenic precursor cells (MPCs) is a key signal for their exit of cell cycle and entry of the myogenic differentiation program. Therefore, identification of the signaling mechanism that activates p38 MAPK during this process is important for the understanding of the regulatory mechanism of muscle regeneration. This article reviews recent findings regarding the role of inflammatory cytokine tumor necrosis factor-α (TNFα) as a key activator of p38 MAPK during myogenesis in an autocrine/paracrine fashion, and the signaling mechanisms that converge upon TNFα converting enzyme (TACE) to release TNFα from differentiating MPCs in response to diverse regenerative stimuli.

Co-activation of nuclear factor-κB and myocardin/serum response factor conveys the hypertrophy signal of high insulin levels in cardiac myoblasts

Hyperinsulinemia contributes to cardiac hypertrophy and heart failure in patients with the metabolic syndrome and type 2 diabetes. Here, high circulating levels of tumor necrosis factor (TNF)-α may synergize with insulin in signaling inflammation and cardiac hypertrophy. We tested whether high insulin affects activation of TNF-α-induced NF-κB and myocardin/serum response factor (SRF) to convey hypertrophy signaling in cardiac myoblasts. In canine cardiac myoblasts, treatment with high insulin (10(-8) to 10(-7) m) for 0-24 h increased insulin receptor substrate (IRS)-1 phosphorylation at Ser-307, decreased protein levels of chaperone-associated ubiquitin (Ub) E3 ligase C terminus of heat shock protein 70-interacting protein (CHIP), increased SRF activity, as well as β-myosin heavy chain (MHC) and myocardin expressions. Here siRNAs to myocardin or NF-κB, as well as CHIP overexpression prevented (while siRNA-mediated CHIP disruption potentiated) high insulin-induced SR element (SRE) activation and β-MHC expression. Insulin markedly potentiated TNF-α-induced NF-κB activation. Compared with insulin alone, insulin+TNF-α increased SRF/SRE binding and β-MHC expression, which was reversed by the NF-κB inhibitor pyrrolidine dithiocarbamate (PDTC) and by NF-κB silencing. In the hearts of db/db diabetic mice, in which Akt phosphorylation was decreased, p38MAPK, Akt1, and IRS-1 phosphorylation at Ser-307 were increased, together with myocardin expression as well as SRE and NF-κB activities. In response to high insulin, cardiac myoblasts increase the expression or the promyogenic transcription factors myocardin/SRF in a CHIP-dependent manner. Insulin potentiates TNF-α in inducing NF-κB and SRF/SRE activities. In hyperinsulinemic states, myocardin may act as a nuclear effector of insulin, promoting cardiac hypertrophy.

Molecular and physiological events in respiratory muscles and blood of rats exposed to inspiratory threshold loading

High-intensity exercise induces oxidative stress and inflammatory events in muscles. Tumor necrosis factor (TNF)-α may alter muscle protein metabolism or promote muscle regeneration. We hypothesized that a program of noninvasive chronic inspiratory loading of different intensities induces a differential pattern of physiological, molecular, and cellular events within rat diaphragms. Antioxidants and TNF-α blockade may influence those events. In the diaphragm, gastrocnemius, and blood of rats exposed to high-intensity inspiratory threshold loads (2 hour every 24 hours for 14 days), with and without treatment with N-acetyl cysteine or infliximab (anti-TNF-α antibody), inflammatory cells and cytokines, superoxide anion production, myogenesis markers, and muscle structure were explored. In all animals, maximum inspiratory pressure (MIP) and body weight were determined. High-intensity inspiratory loading for 2 weeks caused a decline in MIP and body weight, and in the diaphragm induced a reduction in fast-twitch fiber proportions and sizes, whereas inflammatory cells and cytokine levels, including TNF-α immunohistochemical expression, superoxide anion, internal nuclei counts, and markers of myogenesis were increased. Blockade of TNF-α improved respiratory muscle function and structure, and animal weight, and, in the diaphragm, reduced inflammatory cell numbers and superoxide anion production drastically while inducing larger increases in protein and messenger RNA levels and immunohistochemical expression of TNF-α, internal nuclei, and markers of muscle regeneration. Blunting of TNF-α also induced a reduction in blood inflammatory cytokines and superoxide anion production. We conclude that TNF-α synthesized by inflammatory cells or myofibers could have differential effects on muscle structure and function in response to chronic, noninvasive, high-intensity inspiratory threshold loading.

Fold change of nuclear NF-κB determines TNF-induced transcription in single cells

In response to tumor necrosis factor (TNF), NF-κB enters the nucleus and promotes inflammatory and stress-responsive gene transcription. Because NF-κB deregulation is associated with disease, one might expect strict control of NF-κB localization. However, nuclear NF-κB levels exhibit considerable cell-to-cell variability, even in unstimulated cells. To resolve this paradox and determine how transcription-inducing signals are encoded, we quantified single-cell NF-κB translocation dynamics and transcription in the same cells. We show that TNF-induced transcription correlates best with fold change in nuclear NF-κB, not absolute nuclear NF-κB abundance. Using computational modeling, we find that an incoherent feedforward loop, from competition for binding to κB motifs, could provide memory of the preligand state necessary for fold-change detection. Experimentally, we observed three gene-specific transcriptional patterns that our model recapitulates by modulating competition strength alone. Fold-change detection buffers against stochastic variation in signaling molecules and explains how cells tolerate variability in NF-κB abundance and localization.

Sphingosine 1-phosphate axis: a new leader actor in skeletal muscle biology

Sphingosine 1-phosphate (S1P) is a bioactive lipid involved in the regulation of biological processes such as proliferation, differentiation, motility, and survival. Here we review the role of S1P in the biology and homeostasis of skeletal muscle. S1P derives from the catabolism of sphingomyelin and is produced by sphingosine phosphorylation catalyzed by sphingosine kinase (SK). S1P can act either intracellularly or extracellularly through specific ligation to its five G protein-coupled receptors (GPCR) named S1P receptors (S1PR). Many experimental findings obtained in the last 20 years demonstrate that S1P and its metabolism play a multifaceted role in the regulation of skeletal muscle regeneration. Indeed, this lipid is known to activate muscle-resident satellite cells, regulating their proliferation and differentiation, as well as mesenchymal progenitors such as mesoangioblasts that originate outside skeletal muscle, both involved in tissue repair following an injury or disease. The molecular mechanism of action of S1P in skeletal muscle cell precursors is highly complex, especially because S1P axis is under the control of a number of growth factors and cytokines, canonical regulators of skeletal muscle biology. Moreover, this lipid is crucially involved in the regulation of skeletal muscle contractile properties, responsiveness to insulin, fatigue resistance and tropism. Overall, on the basis of these findings S1P signaling appears to be an appealing pharmacological target for improving skeletal muscle repair. Nevertheless, further understanding is required on the regulation of S1P downstream signaling pathways and the expression of S1PR. This article will resume our current knowledge on S1P signaling in skeletal muscle, hopefully stimulating further investigation in the field, aimed at individuating novel molecular targets for ameliorating skeletal muscle regeneration and reducing fibrosis of the tissue after a trauma or due to skeletal muscle diseases.

Overexpression of TNF-α in mitochondrial diseases caused by mutations in mtDNA: evidence for signaling through its receptors on mitochondria

Mitochondrial diseases (MDs) are heterogeneous disorders due to impaired respiratory chain function causing defective ATP production. Although the disruption of oxidative phosphorylation is central to the MD pathophysiology, other factors may contribute to these disorders. We investigated the expression and the cellular localization of TNF-α and its receptors, TNFR1 and TNFR2, in muscle biopsies from 15 patients with mitochondrial respiratory chain dysfunction. Our data unambiguously demonstrate that TNF-α is expressed in muscle fibers with abnormal focal accumulations of mitochondria, so-called ragged red fibers, and is delivered to mitochondria where both receptors are localized. Moreover TNF receptors are differentially regulated in patients' muscle in which the expression levels of TNFR1 mRNA are decreased and those of TNFR2 mRNA are increased compared with controls. These findings suggest for the first time that TNF-α could exert a direct effect on mitochondria via its receptors.

Regulation of IRS1/Akt insulin signaling by microRNA-128a during myogenesis

Skeletal muscle possesses a strong ability to regenerate following injury, a fact that has been largely attributed to satellite cells. Satellite cells are skeletal muscle stem cells located beneath the basal lamina of the myofiber, and are the principal cellular source of growth and regeneration in skeletal muscle. MicroRNAs (miRNAs) play key roles in modulating several cellular processes by targeting multiple mRNAs that comprise a single or multiple signaling pathway. Several miRNAs have been shown to regulate satellite cell activity, such as miRNA-489, which functions to maintain satellite cells in a quiescent state. Although muscle-specific miRNAs have been identified, many of the molecular mechanisms that regulate myogenesis that are regulated by miRNAs still remain unknown. In this study, we have shown that miR-128a is highly expressed in brain and skeletal muscle, and increases during myoblast differentiation. MiR-128a was found to regulate the target genes involved in insulin signaling, which include Insr (insulin receptor), Irs1 (insulin receptor substrate 1) and Pik3r1 (phosphatidylinositol 3-kinases regulatory 1) at both the mRNA and protein level. Overexpression of miR-128a in myoblasts inhibited cell proliferation by targeting IRS1. By contrast, inhibition of miR-128a induced myotube maturation and myofiber hypertrophy in vitro and in vivo. Moreover, our results demonstrate that miR-128a expression levels are negatively controlled by tumor necrosis factor α (TNF-α). TNF-α promoted myoblast proliferation and myotube hypertrophy by facilitating IRS1/Akt signaling via a direct decrease of miR-128a expression in both myoblasts and myotubes. In summary, we demonstrate that miR-128a regulates myoblast proliferation and myotube hypertrophy, and provides a novel mechanism through which IRS1-dependent insulin signaling is regulated in skeletal muscle.

Oxygen consumption and usage during physical exercise: the balance between oxidative stress and ROS-dependent adaptive signaling

The complexity of human DNA has been affected by aerobic metabolism, including endurance exercise and oxygen toxicity. Aerobic endurance exercise could play an important role in the evolution of Homo sapiens, and oxygen was not important just for survival, but it was crucial to redox-mediated adaptation. The metabolic challenge during physical exercise results in an elevated generation of reactive oxygen species (ROS) that are important modulators of muscle contraction, antioxidant protection, and oxidative damage repair, which at moderate levels generate physiological responses. Several factors of mitochondrial biogenesis, such as peroxisome proliferator-activated receptor-γ coactivator 1α (PGC-1α), mitogen-activated protein kinase, and SIRT1, are modulated by exercise-associated changes in the redox milieu. PGC-1α activation could result in decreased oxidative challenge, either by upregulation of antioxidant enzymes and/or by an increased number of mitochondria that allows lower levels of respiratory activity for the same degree of ATP generation. Endogenous thiol antioxidants glutathione and thioredoxin are modulated with high oxygen consumption and ROS generation during physical exercise, controlling cellular function through redox-sensitive signaling and protein-protein interactions. Endurance exercise-related angiogenesis, up to a significant degree, is regulated by ROS-mediated activation of hypoxia-inducible factor 1α. Moreover, the exercise-associated ROS production could be important to DNA methylation and post-translation modifications of histone residues, which create heritable adaptive conditions based on epigenetic features of chromosomes. Accumulating data indicate that exercise with moderate intensity has systemic and complex health-promoting effects, which undoubtedly involve regulation of redox homeostasis and signaling.

Cytokine response of cultured skeletal muscle cells stimulated with proinflammatory factors depends on differentiation stage

Myoblast proliferation and myotube formation are critical early events in skeletal muscle regeneration. The attending inflammation and cytokine signaling are involved in regulation of skeletal muscle cell proliferation and differentiation. Secretion of muscle-derived cytokines upon exposure to inflammatory factors may depend on the differentiation stage of regenerating muscle cells. Cultured human myoblasts and myotubes were exposed to 24-hour treatment with tumor necrosis factor (TNF)-α or lipopolysaccharide (LPS). Secretion of interleukin 6 (IL-6), a major muscle-derived cytokine, and interleukin 1 (IL-1), an important regulator of inflammatory response, was measured 24 hours after termination of TNF-α or LPS treatment. Myoblasts pretreated with TNF-α or LPS displayed robustly increased IL-6 secretion during the 24-hour period after removal of treatments, while IL-1 secretion remained unaltered. IL-6 secretion was also increased in myotubes, but the response was less pronounced compared with myoblasts. In contrast to myoblasts, IL-1 secretion was markedly stimulated in LPS-pretreated myotubes. We demonstrate that preceding exposure to inflammatory factors stimulates a prolonged upregulation of muscle-derived IL-6 and/or IL-1 in cultured skeletal muscle cells. Our findings also indicate that cytokine response to inflammatory factors in regenerating skeletal muscle partially depends on the differentiation stage of myogenic cells.

Sphingosine kinase/sphingosine 1-phosphate axis: a new player for insulin-like growth factor-1-induced myoblast differentiation

Background

Insulin-like growth factor-1 (IGF-1) is the most important physiological regulator of skeletal muscle progenitor cells, which are responsible for adult skeletal muscle regeneration. The ability of IGF-1 to affect multiple aspects of skeletal muscle cell biology such as proliferation, differentiation, survival and motility is well recognized, although the molecular mechanisms implicated in its complex biological action are not fully defined. Since sphingosine 1-phosphate (S1P) has recently emerged as a key player in skeletal muscle regeneration, we investigated the possible involvement of the sphingosine kinase (SK)/S1P receptor axis on the biological effects of IGF-1 in murine myoblasts.

Methods

RNA interference, chemical inhibition and immunofluorescence approaches were used to assess the role of the SK/S1P axis on the myogenic and mitogenic effects of IGF-1 in C2C12 myoblasts.

Results

We show that IGF-1 increases SK activity in mouse myoblasts. The effect of the growth factor does not involve transcriptional regulation of SK1 or SK2, since the protein content of both isoforms is not affected; rather, IGF-1 enhances the fraction of the active form of SK. Moreover, transactivation of the S1P₂ receptor induced by IGF-1 via SK activation appears to be involved in the myogenic effect of the growth factor. Indeed, the pro-differentiating effect of IGF-1 in myoblasts is impaired when SK activity is pharmacologically inhibited, or SK1 or SK2 are specifically silenced, or the S1P₂ receptor is downregulated. Furthermore, in this study we show that IGF-1 transactivates S1P₁/S1P₃ receptors via SK activation and that this molecular event negatively regulates the mitogenic effect elicited by the growth factor, since the specific silencing of S1P₁ or S1P₃ receptors increases cell proliferation induced by IGF-1.

Conclusions

We demonstrate a dual role of the SK/S1P axis in response to myoblast challenge with IGF-1, that likely is important to regulate the biological effect of this growth factor. These findings add new information to the understanding of the mechanism by which IGF-1 regulates skeletal muscle regeneration.

The central role of myostatin in skeletal muscle and whole body homeostasis

Myostatin is a powerful negative regulator of skeletal muscle mass in mammalian species. It plays a key role in skeletal muscle homeostasis and has now been well described since its discovery. Myostatin is capable of inducing muscle atrophy via its inhibition of myoblast proliferation, increasing ubiquitin-proteasomal activity and downregulating activity of the IGF-Akt pathway. These well-recognized effects are seen in multiple atrophy causing situations, including injury, diseases such as cachexia, disuse and space flight, demonstrating the importance of the myostatin signalling mechanism. Based on this central role, significant work has been pursued to inhibit myostatin's actions in vivo. Importantly, several new studies have uncovered roles for myostatin distinct from skeletal muscle size. Myostatin has been suggested to play a role in cardiomyocyte homeostasis, glucose metabolism and adipocyte proliferation, all of which are examined in detail below. Based on these effects, myostatin inhibition has potential to be widely utilized in many Western diseases such as chronic obstructive pulmonary disease, type II diabetes and obesity. However, if myostatin inhibitors are to successfully translate from bench-top to bedside in the near future, awareness must be raised on these non-traditional effects of myostatin away from skeletal muscle. Indeed, further research into these novel areas is required.

The effect of the NF-kappa B inhibitors curcumin and lactacystin on myogenic differentiation of rhabdomyosarcoma cells

Rhabdomyosarcoma is a soft tissue sarcoma mainly seen in children. Despite considerable progress within the last few years, therapeutic approaches for this type of tumor are still limited. The respective tumor cells originate from myogenic precursor cells and are characterized by a blockade in their differentiation program. Interestingly, there is a direct inverse correlation between the differentiation status of a specific rhabdomyosarcoma cell and its metastatic potential. Thus, here, we tested whether the ubiquitous transcription factor NF-κB, which regulates myogenic differentiation and is also a promising therapeutic target in the treatment of other types of tumors, might be an interesting candidate for the development of novel rhabdomyosarcoma treatment strategies. For this purpose, we analyzed NF-κB activity (classical pathway) in myoblasts with different differentiation potential, specifically in three different rhabdomyosarcoma cell lines. In addition, we inhibited NF-κB activity in these cells and analyzed the effects on myogenic differentiation. We show that after the induction of differentiation, NF-κB activity declines rapidly in normal myoblasts, but only slightly in rhabdomyosarcoma cells. However, after treatment of the cells with two different small-molecule NF-κB-inhibiting compounds, the IKK inhibitor curcumin and the proteasome inhibitor lactacystin, we found that neither curcumin nor lactacystin promoted myogenic differentiation in either normal myoblasts or rhabdomyosarcoma cells. Taken together, our data suggest that treatment with curcumin or lactacystin might not be a suitable approach in the treatment of rhabdomyosarcoma.

Growth factor and cytokine interactions in myogenesis. Part I. The effect of TNF-alpha and IFN-gamma on IGF-I-dependent differentiation in mouse C2C12 myogenic cells

The aim of this study was to examine the potential interactions of IGF-I with TNF-alpha and IFN-gamma with regard to regulation of the myogenesis and proliferative potential of mouse C2C12 myoblasts. The stimulation of myogenesis by IGF-I (30 nmol/l) was manifested by an enhanced myoblast fusion and expression of myosin heavy chain (MHC) during the first 3 days of differentiation. IGF-I-dependent fusion and MHC expression was reduced by TNF-alpha and IFN-gamma. Both cytokines prevented the stimulatory effect of IGF-I on MyoD expression with minor modification of the myogenin level. Both TNF-alpha and IFN-gamma activated the expression of cyclin A in myoblasts restimulated to proliferation; however, when used in combination with IGF-I these cytokines prevented the rise in cyclin A induced by growth factor.

IN CONCLUSION

i) TNF-alpha and IFN-gamma reduce IGF-I-dependent myogenesis which was manifested by the reduction of myoblast fusion and MHC cellular levels, ii) Molecular mechanisms of inhibitory action of TNF-alpha and IFN-gamma on IGF-I-mediated differentiation involve a decrease in MyoD whereas myogenin level plays a minor role, iii) TNF-alpha and IFN-gamma increase the proliferative potential of myoblasts; however, they reduced the mitogenic effect of IGF-I, manifested by a decrease of IGF-I-stimulated cyclin A expression in myoblasts reinduced to proliferation. Interactions among IGF-I and proinflammatory cytokines are therefore important to establish a number of myoblasts and the onset of myogenesis during muscle regeneration.

Plasma from septic shock patients induces loss of muscle protein

Introduction

ICU-acquired muscle weakness commonly occurs in patients with septic shock and is associated with poor outcome. Although atrophy is known to be involved, it is unclear whether ligands in plasma from these patients are responsible for initiating degradation of muscle proteins. The aim of the present study was to investigate if plasma from septic shock patients induces skeletal muscle atrophy and to examine the time course of plasma-induced muscle atrophy during ICU stay.

Methods

Plasma was derived from septic shock patients within 24 hours after hospital admission (n = 21) and healthy controls (n = 12). From nine patients with septic shock plasma was additionally derived at two, five and seven days after ICU admission. These plasma samples were added to skeletal myotubes, cultured from murine myoblasts. After incubation for 24 hours, myotubes were harvested and analyzed on myosin content, mRNA expression of E3-ligase and Nuclear Factor Kappa B (NFκB) activity. Plasma samples were analyzed on cytokine concentrations.

Results

Myosin content was approximately 25% lower in myotubes exposed to plasma from septic shock patients than in myotubes exposed to plasma from controls (P < 0.01). Furthermore, patient plasma increased expression of E3-ligases Muscle RING Finger protein-1 (MuRF-1) and Muscle Atrophy F-box protein (MAFbx) (P < 0.01), enhanced NFκB activity (P < 0.05) and elevated levels of ubiquitinated myosin in myotubes. Myosin loss was significantly associated with elevated plasma levels of interleukin (IL)-6 in septic shock patients (P < 0.001). Addition of antiIL-6 to septic shock plasma diminished the loss of myosin in exposed myotubes by approximately 25% (P < 0.05). Patient plasma obtained later during ICU stay did not significantly reduce myosin content compared to controls.

Conclusions

Plasma from patients with septic shock induces loss of myosin and activates key regulators of proteolysis in skeletal myotubes. IL-6 is an important player in sepsis-induced muscle atrophy in this model. The potential to induce atrophy is strongest in plasma obtained during the early phase of human sepsis.

The sphingosine kinase activator K6PC-5 stimulates C2C12 myoblast differentiation

Previously, K6PC-5, a synthetic derivative of ceramide, was demonstrated to activate sphingosine kinase (SK)-1 in keratinocytes. In this study its potential biological effect in mouse myoblasts was examined. The obtained results show that K6PC-5 promotes myogenic differentiation by enhancing myogenic marker expression, differentiation index and fusion index. Interestingly, its biological action was prevented by pharmacological inhibition of SK or S1P2 receptor, in full agreement with their recognized role in myoblast differentiation. This is the first evidence that pharmacological activation of SK accelerates myogenesis and suggests that this new therapeutic strategy could be possibly employed in skeletal muscle disorders where muscle regeneration is deficient.

αB-crystallin, a small heat shock protein, modulates NF-κB activity in a phosphorylation-dependent manner and protects muscle myoblasts from TNF-α induced cytotoxicity

αB-crystallin, a member of the small heat shock protein family, has been implicated in various biological functions including response to heat shock, differentiation and apoptosis, the mechanisms of which have not been well understood. Myoblasts, the precursor cells in muscle regeneration, when subjected to growth factor deprivation differentiate to form myotubes or undergo apoptosis. During differentiation, myoblasts express elevated levels of αB-crystallin as well as TNF-α but the connecting link between these proteins in cell signaling is not clearly understood. We have therefore investigated the role of αB-crystallin in TNF-α induced regulation of NF-κB. We demonstrate that in response to TNF-α treatment, αB-crystallin associates with IKKβ and activate its kinase activity, facilitating the degradation of phosphorylated I-kBα, a prime step in NF-κB activation. Reducing the level of αB-crystallin using the RNAi approach reduces the translocation of p65, further confirming the role of αB-crystallin in NF-κB activation. Our study shows that the ability of αB-crystallin to activate NF-κB depends on its phosphorylation status. The present study shows that αB-crystallin-dependent NF-κB activation protects myoblasts from TNF-α induced cytoxicity by enhancing the expression of the anti-apoptotic protein, Bcl 2. Thus, our study identifies yet another mechanism by which αB-crystallin exerts its anti-apoptotic activity.